Liquid Lenses Make Better Beams

AMES, Iowa -- To micromachine transistors, integrated chips and similar miniature devices requires lasers with narrow beams and small spot sizes that etch and cut cleanly and precisely. If drilling is done, the entrance and exit holes must be as close to identical as possible for maximum chip performance.

A research team led by Palaniappa Molian of Iowa State University's department of mechanical engineering seems to have found a way to achieve these goals, using lenses made of liquid instead of glass.

Scanning-electron micrographs show that the entrance and exit holes drilled by lasers with conventional glass lenses (A and B) are larger, cruder and less uniform than holes
made by lasers using a liquid lens (C and D).
The team described the technique in the March 12 issue of Applied Physics Letters. A liquid with a nonlinear refractive index -- in this case, carbon disulfide -- was placed in a custom-made fused-silica glass beaker that had a minimal index of refraction. When a high-intensity light from a laser passed through the 1/4-in.-thick, 1-in.-diameter column of liquid, its rays converged, much like light passing through a standard glass lens.

The tiny subcomponents of transistors and integrated circuits may be etched more effectively with lasers that use a liquid as a lens instead of a glass optic, say researchers. Liquid optics produce narrower beams with smaller spot sizes, as seen in these diagrams comparing the traditional method (a) with liquid optics (b). All diagrams and photos courtesy of Palaniappa Molian of Iowa State University.
By manipulating the depth of carbon disulfide, the scientists found that they could make the liquid's refractive index change. And by varying this in conjunction with the incoming laser beam's intensity and pulse width, they could control the outgoing beam's diameter and focal point. "When the depth is just perfect, the raw beam comes out very well-focused. The liquid is now a lens," team member Diwakar Ramanathan said.

The emitted beam drilled holes in a steel sample that were one-third the size of holes created by conventional glass lenses. The holes also were cleaner, with minimal tapering between entrance and exit.

Ramanathan cited an additional benefit: The liquid made the beam split into numerous smaller beamlets, allowing users the potential to machine several areas simultaneously with a single laser.

He cautioned that the researchers need to study this aspect further and to learn methods of controlling it. They also are experimenting with nonlinear liquids other than carbon disulfide, which is highly toxic and volatile.

Solutions may come from unexpected sources. Ramanathan said that team members have successfully experimented with Chinese tea and spinach juice.